This invention relates to the measurement of the rate of flow of fluid, and more particularly to the measurement of the rate of flow of a compressible fluid.
Differential pressure devices, such as orifices, have long been used to measure the flowrates of fluids such as natural gas. In this method the measuring device comprises a conduit, which is usually circular in construction, the interior of which is sealed with a baffle plate having an orifice of known size and shape therein. The measurement of flowrate is accomplished by passing the fluid through the conduit and through the orifice. The pressure is measured on both the upstream and downstream sides of the baffle plate. Knowing both the pressure differential and the orifice size and geometry, the flowrate can be calculated using the orifice equation, which is well known in the art.
The major drawback to this method is that the orifice equation is dependent upon the size and shape of the orifice. If the orifice is different from that assumed by the equation, the resulting flowrate is inaccurate. In practice, the orifice increases in size and changes in shape as it ages due to wear from the passage of fluids. The usual way of assuring the accuracy of flowrate measurements is to change the baffle plate frequently to minimize variations in the size and shape of the orifice.
The major consequence of a change in the orifice size through wear is to indicate a smaller flowrate than actually exists. Thus if the orifice increases, the pressure differential will drop, even though the flowrate remains constant. In such industries as natural gas production and transmission small errors in measurement of the flowrate can result in large economic losses. These losses have been greatly magnified in recent years as the value of natural gas has increase manifold.
A further problem in using the standard orifice equation is that it assumes that there is no temperature differential across the orifice plate. This assumption is correct only for fluids that are relatively incompressible under normal conditions. For fluids such as gases the compressibility, and thus the temperature differential, cannot be ignored where highly accurate measurement is desired. Failure to recognize and take this effect into account has rendered flowrate measurements based upon the standard orifice equation significantly inaccurate.